EL2250C, EL2450C
125MHz Single Supply Dual/Quad Op Amps
Applications Information
Product Description
Supply Voltage Range and Single-Supply
Operation
The EL2250C/EL2450C are part of a family of the
industries fastest single supply operational amplifiers.
Connected in voltage follower mode, their -3dB band-
width is 125MHz while maintaining a 275 V/µs slew
rate. With an input and output common mode range that
includes ground, these amplifiers were optimized for
single supply operation, but will also accept dual sup-
plies. They operate on a total supply voltage range as
low as +2.7V or up to +12V. This makes them ideal for
+3V applications, especially portable computers.
The EL2250C/EL2450C have been designed to operate
with supply voltages having a span of greater than 2.7V,
and less than 12V. In practical terms, this means that the
EL2250C/EL2450C will operate on dual supplies rang-
ing from ±1.35V to ±6V. With a single-supply, the
EL2250C/EL2450C will operate from +2.7V to +12V.
Performance has been optimized for a single +5V
supply.
Pins 8 and 4 are the power supply pins on the EL2250C.
The positive power supply is connected to pin 8. When
used in single supply mode, pin 4 is connected to
ground. When used in dual supply mode, the negative
power supply is connected to pin 4.
While many amplifiers claim to operate on a single sup-
ply, and some can sense ground at their inputs, most fail
to truly drive their outputs to ground. If they do succeed
in driving to ground, the amplifier often saturates, caus-
ing distortion and recovery delays. However, special
circuitry built into the EL2250C/EL2450C allows the
output to follow the input signal to ground without
recovery delays.
Pins 4 and 11 are the power supply pins on the
EL2450C. The positive power supply is connected to pin
4. When used in single supply mode, pin 11 is connected
to ground. When used in dual supply mode, the negative
power supply is connected to pin 11.
Power Supply Bypassing And Printed Circuit
Board Layout
As supply voltages continue to decrease, it becomes nec-
essary to provide input and output voltage ranges that
can get as close as possible to the supply voltages. The
EL2250C/EL2450C have an input voltage range that
includes the negative supply and extends to within 1.2V
of the positive supply. So, for example, on a single +5V
supply, the EL2250C/EL2450C have an input range
which spans from 0V to 3.8V.
As with any high-frequency device, good printed circuit
board layout is necessary for optimum performance.
Ground plane construction is highly recommended.
Lead lengths should be as short as possible. The power
supply pins must be well bypassed to reduce the risk of
oscillation. The combination of a 4.7µF tantalum capac-
itor in parallel with a 0.1 µF ceramic capacitor has been
shown to work well when placed at each supply pin. For
single supply operation, where the GND pin is con-
nected to the ground plane, a single 4.7µF tantalum
capacitor in parallel with a 0.1µF ceramic capacitor
across the VS+ and GND pins will suffice.
The output range of the EL2250C/EL2450C is also quite
large. It includes the negative rail, and extends to within
1V of the top supply rail with a 1kW load. On a +5V sup-
ply, the output is therefore capable of swinging from 0V
to +4V. On split supplies, the output will swing ±4V. If
the load resistor is tied to the negative rail and split sup-
plies are used, the output range is extended to the
negative rail.
For good AC performance, parasitic capacitance should
be kept to a minimum. Ground plane construction
should be used. Carbon or Metal-Film resistors are
acceptable with the Metal-Film resistors giving slightly
less peaking and bandwidth because of their additional
series inductance. Use of sockets, particularly for the SO
package should be avoided if possible. Sockets add par-
asitic inductance and capacitance which will result in
some additional peaking and overshoot.
Choice Of Feedback Resistor, RF
The feedback resistor forms a pole with the input capac-
itance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
peaking in the frequency domain. Therefore, RF has
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